PROJECT SUMMARY/ABSTRACT Therapeutics to prevent, delay and treat Alzheimer?s disease (AD) remain an unmet need. Proposed herein is a regenerative medicine, systems biology approach that targets the regenerative system of the brain while simultaneously activating systems to reduce burden of AD pathology. Allopregnanolone (Allo) is a pleiotropic neurosteroid that in preclinical discovery models of AD and aging promotes neurogenesis, restores cognitive function and reduces burden of AD pathology. Mechanisms by which Allo promotes neural stem cell regeneration and restoration of cognitive function are extensively characterized with a large margin of safety. Importantly, Allo promotes regeneration of human neural stem cells in vitro. Allo is a low molecular weight neurosteroid endogenous to the brain that is blood brain barrier penetrant with abundant existing safety data in animals and humans. Completed National Institute on Aging (NIA) Phase 1 clinical trial of Allo in persons diagnosed with MCI due to AD or mild AD, indicates that the regenerative treatment regimen of once per week via intravenous infusion is well tolerated with no indications of Allo-related adverse events. MRI brain imaging for regenerative surrogate markers and cognitive testing were well tolerated and feasible in this early AD cohort. Safety and tolerability findings in women and men are consistent with outcomes of IND-enabling chronic toxicology in two species indicating no adverse outcomes following 24 weeks of once per week Allo exposure at doses exceeding those to be tested in humans by 10-fold. Based on a foundation of discovery and mechanistic preclinical research, IND-enabling studies and Phase 1 clinical development in women and men, we propose a delayed start Phase 2 clinical trial of Allo administered in a regenerative treatment regimen for 18 months, which includes a placebo-controlled period of 12 months followed by a delayed-start (open label) period of 6 months. To advance clinical development, three specific aims are proposed. Aim 1 is designed to conduct a Phase 2, randomized, placebo-controlled, delayed start group, proof of concept clinical trial of Allo in APOEe4 positive participants diagnosed with mild AD. The primary outcome measure will be rate of change in ADAS-cog14 score after 12 months. Secondary analyses will assess change from baseline to 12 months on activities of daily living assessed by ADCS-iADL, MRI volumetric outcomes, and on cognitive function as determined by CANTAB AD battery, MMSE, and CDR-SB. Aim 1 exploratory analyses will assess cognitive clinical and functional outcomes during the delayed-start period (12-18 months). Aim 2 is exploratory and designed to develop surrogate MRI-based biomarkers of hippocampal regeneration and connectivity. Aim 3 is exploratory and is designed to establish a blood-based predictive biomarker of regenerative responders and non-responders. To be explored are Allo-induced regeneration of iPSC-derived neural stem cells and mitochondrial respiration. Secondary objective is to determine the cellular population with greatest predictive accuracy using participant derived iPSCs / neural stem cells, peripheral blood mononuclear cells and CD34+ cells.